"I obviously had the foundation of David in place, and I had an idea where he would go. And because David is such an expressive and theatrical sort of character, I wanted to make sure that Walter is not.

"So he is plain and sort of bland and neutral, and he does not have any human characteristics in his programming. He does not have any of his own motives at play, and he is just there to serve and protect the crew and the ship."

At the Ham Yard Hotel in London, the German-born Irish actor was a cheerful fellow at interviews, quick to laugh and joke around.

He said working with Scott was a "blast".

"This was probably the most fun I have had on any film, and I think Ridley had a really good time as well," said Fassbender.

"Ridley has great confidence and passion in making films, and he has always got this sort of mischievousness, curiosity and youthfulness about him, the way he goes about shooting a film.

"That permeates through the crew, and you can just tell that everybody is happy to be working on a Ridley Scott film. I would love to do another one, for sure."

There was a lot of joking around on the set, Fassbender said - Crudup and McBride did their best to make him break up laughing during a scene.

"I am not great at keeping a straight face. Danny is a master of comedy, so it is not too difficult for him," Fassbender said.

"I find that when I am trying to make other people laugh, I end up laughing, because they sort of bounce it back at me."

He remembers seeing the original Alien movie when he was 10.

"I did not speak a lot and was just sort of transfixed on the television screen, and some feeling was happening," he said, laughing.

"I was terrified, but I did not want my parents to see that, or they would send me to bed. And it is one of those films that I remember what it was like sitting in the room. I remember clearly that experience."

SURFING

Shooting Alien: Covenant in Sydney, Australia, suited him perfectly as he could indulge in his passion for surfing.

"I loved it there. It is such a great town with great people. And I enjoyed being by the beach. I went to loads of them, such as Freshwater, Manly and Bondi.

"It was great, I loved it. I did a little bit of walking, but most of the time I was trying to surf," Fassbender said.

He described himself as homeless, as he has no permanent base.

"If I am moving around, I do not stay in hotels that much. So for example, filming in Sydney, the priority for me was to find an apartment or a house as quickly as I could.

"I found a really nice place in Tamarama, and that was home then for four months," he said.

"That is kind of the same wherever I go - I just make that place my home. I put food in the fridge and that is kind of it, really.

"I do not have a lot of possessions that I carry around with me, other than clothes. I do like to travel as light as possible. And I can cook."

What can he cook?

"Whatever. Somewhere like Australia, it is perfect for barbecues, so I barbecue. Most times, fish, meat, whatever, pasta if I want something quick, simple things.

"I would not go about making a curry or anything like that."

Fassbender just turned 40 last month, and he celebrated with his family and friends in Ireland.

"It feels good and bad," he said, laughing.

"I feel good about who I am, and I feel pretty good in my life. But I am getting older," he said.

"My sister arranged a surprise birthday party for me, which was not a surprise, because she had to tell me so that I was there for it.

"But it was cool. There were loads of friends, some of them I have known since I was five."